The present invention relates to a device and a method for detecting ammonia, especially an NH3 slip, in a catalytic converter arrangement.
For reducing the NOx emissions from lean-mixture-driven internal combustion engines, especially diesel engines, an SCR catalytic converter (xe2x80x9cselective catalytic reductionxe2x80x9d) may be used for the exhaust treatment. In this respect, reference should be made to D. Schxc3x6ppe, et al., xe2x80x9cA Regulated Exhaust Treatment System for Meeting Future Emissions Threshold Values in Diesel Engines,xe2x80x9d Progress Reports, VDI, Series 12, No. 267, Vol. 1 (1996), 17th International Vienna Engine Symposium, pp. 332-353, in which the mode of operation and the construction of an SCR catalytic converter of this type is discussed.
Summarizing, in an SCR catalytic converter, a selective catalytic reduction of NOx to N2 takes place, NH3 (ammonia) acting as a reducing agent, which can be obtained from a liquid urea solution in a hydrolysis catalytic converter upstream of the SCR catalytic converter.
However, in the operation of a system of this type for an exhaust treatment using an SCR catalytic converter, the problem arises that too much urea may be apportioned out, so that unconverted ammonia reaches the ambient air along with the exhaust via the SCR catalytic converter. If this NH3 emission is to be suppressed or monitored, an NH3 sensor by be required.
The exemplary device and method of the present inventions are believed to have the advantage that expensive sensors that specifically target NH3 and that are technically difficult to realize are not used for detecting ammonia or NH3 emissions. As a result, it is possible in an advantageous manner for a broader spectrum of sensor concepts or sensor materials to be considered for use, i e., for detecting ammonia, it is possible to use available gas sensors, especially those that do not react selectively to specific oxidizable gases or gas components, which may represent a significant cost advantage and a simplification with respect to the technology and manufacture of the gas sensors that can be used. In particular, the spectrum of gas sensors that can be used for detecting ammonia can be broadened significantly, in an advantageous manner, to include gas sensors having metallic electrodes, specifically Pt/Au systems, which may already be used as HC sensors.
As a result of the fact that the known SCR catalytic converter, via an upstream first means, has supplied to it an already at least substantially, preferably completely, pre-cleaned or pre-oxidized gas or exhaust gas, this gas already no longer contains any oxidizable or combustible gas components. In particular, gas components HC and CO are no longer contained in this pre-cleaned gas.
Upstream of the feed to the SCR catalytic converter, via a second gas supply device, ammonia is advantageously added to the gas that has been pre-cleaned in this manner, so that in the SCR catalytic converter no oxidizable gas exists apart from NH3. In the event that the totality of the supplied NH3 is not used up in the SCR catalytic converter for reducing the NOx that is present, an NH3 breakthrough or NH3 emission arises in the SCR catalytic converter, i.e., the gas emerging from the SCR catalytic converter contains ammonia. However, since this ammonia is the only oxidizable gas component contained in the gas, it is not necessary, for detecting the ammonia, to use a means that is specifically targeted to NH3, especially a gas sensor that specifically targets ammonia. A simple sensor suffices that is generally, or across a broad spectrum, sensitive to oxidizable gas components, as is already generally known.
Thus it is believed to be especially advantageous if the gas to be supplied to the SCR catalytic converter before being mixed with ammonia is pre-treated in an available oxidation catalytic converter, which carries out the at least substantial oxidation or pre-cleaning of the gas to be supplied to the SCR catalytic converter.
Supplying the ammonia to the pre-cleaned or pre-oxidized gas, which is to be supplied to the SCR catalytic converter and which, for example, emerges from the oxidation catalytic converter, is carried out in a very advantageous manner using a generally known gas supply device, which includes a hydrolysis catalytic converter for producing ammonia from a chemical compound, especially urea, a dosing system for the dosed and controlled adding of ammonia to the pre-oxidized gas, and an appropriate supply line (e.g., the at least one of the ammonia and the NH3 slip emission is releasable from the at least one of the chemical compound and the urea in the hydrolysis catalytic converter).
Furthermore, it is believed to be advantageous if the second arrangement, device or structure for detecting the oxidizable gas components emerging from the SCR catalytic converter is connected to a processing unit, which is connected via a control unit and a control line to the gas supply device for supplying ammonia to the pre-oxidized gas. In this manner, the addition of ammonia to the gas supplied to the SCR catalytic converter can advantageously be controlled, or, in the form of a closed feedback circuit, can be regulated as a function of the NH3 that is actually consumed for the reduction of NOx, such that the quantity of the unused ammonia emerging from the SCR catalytic converter is minimized or is reduced virtually to zero.